1. Field of the Invention
The present invention relates to an optical communications system and, more particularly, to an optical communication apparatus that modulates the intensity and phase of an optical signal to be transmitted, as well as to a quantum key distribution system using the optical communication apparatus.
2. Description of the Related Art
In the present circumstances where the rapid growth in internet traffic continues, one of the most significant challenges is to enhance transmission bandwidth or transmission speed. This is no exceptional matter with trunk optical networks. Various research institutes and laboratories are working toward an increase in the data transmission speed per carrier in order to expand transmission capacities by utilizing the existing infrastructures without adding transmission lines and repeaters. Data transmission speeds continue to increase also in practical commercial transmission systems.
In general optical communication technologies, since binary amplitude shift keying (ASK) is principally used, a dominant approach to the enhancement of the transmission speed per carrier is to reduce the time slot per bit. However, since around the time the data transmission speed per carrier exceeded 10 Gb/s, it has been difficult to accomplish higher speed only by using ASK.
One of the reasons for this is deterioration in waveform due to wavelength dispersion, which is typical of optical transmission lines. The wavelength dispersion is a phenomenon such that a propagation delay occurring in a transmission line varies depending on the wavelength of signal light. Since a signal-light spectrum has a specific range of wavelengths, a short-wavelength component and a long-wavelength component of the same signal light accumulate different wavelength dispersion values during propagation. This accumulated dispersion after propagation results in a difference in propagation delay, that is, causes waveform distortion. On comparison using an ASK signal, since the signal-light spectrum changes in proportion to the modulation rate, the waveform distortion due to wavelength dispersion becomes greater proportionately as the data transmission speed rises. On the other hand, a time slot becomes shorter proportionately as the data transmission speed rises. Therefore, even if signals suffer the same waveform distortion (that is, signals have the same propagation-delay differences), a higher-speed signal is more affected. For the reasons above, it is said that in ASK, the propagation characteristics deteriorate in proportion to the square of the data transmission speed.
As described above, since it has been difficult to achieve higher speed only by using ASK, another approach is attracting attention, which is a technique for enhancing transmission bandwidth by using a multivalued signal with the increased number of signal states per time slot. For example, four-state modulation of the phase of light may be employed to enhance transmission bandwidth, which is described in Griffin, R. A., et al., “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration,” OFC 2002, PD-FD6.
As another example, multiple digital bits per symbol (for a carrier and a time slot) may be employed by modulating both the intensity and phase of light (APSK: Amplitude Phase Shift Keying), which is described in the following papers:                S. Hayase et al. “Proposal of 8-State per Symbol (Binary ASK and QPSK) 30-Gbit/s Optical Modulation/Demodulation Scheme,” ECOC 2003, Th.2.6.4; and        Ohm, M., et al. “Quaternary Optical ASK-DPSK and Receivers With Direct Detection,” IEEE Photonics Technology Letters, Vol. 15, No. 1, p. 159.        
FIG. 1 is a block diagram showing a schematic configuration of a transmitter that generates an APSK signal, described in S. Hayase et al. A light source 1401 generates continuous wave (CW) light, which is subjected at a first-stage Mach-Zehnder (MZ) modulator 1402 to binary phase shift keying (PSK) with two phases of 0 and π, and is subsequently subjected at a second-stage phase modulator 1403 to binary PSK with two phases of 0 and π/2. Thereby, a PSK signal having four states of 0, π/2, π, and 3π/2 can be obtained. Further, a MZ modulator 1404 performs two-level modulation also in the intensity direction on the four-state PSK signal, thereby generating an eight-state (3 bits/symbol) APSK signal. Furthermore, in this example, a MZ modulator 1405 is also used to return to zero (RZ) each bit.
However, a multivalued signal such as the APSK signal has an increased number of states per symbol on the one hand, but has a reduced distance between signal points on I-Q plane on the other hand. Therefore, if an attempt is made to obtain the same code error rate as that achieved by a binary modulation scheme such as ASK or BPSK, a large carrier-to-noise ratio (CNR) is needed.
However, the use of modulators at multiple stages in generating an APSK signal, as in the transmitter of S. Hayase et al., increases loss and therefore reduces the intensity of output light, that is, the power of carrier. Here, although the intensity of transmitted light can be set high by using an optical amplifier, it is impossible to avoid degradation of the signal-to-nose ratio (SNR) at the time of signal transmission. Incidentally, U.S. Pat. No. 7,023,601 discloses a structure that realizes a single MZ modulator in place of multistage-connected modulators.
In addition, quantum key distribution (QKD) technology can be included in the list of technologies employing such a modulation scheme of modulating both the intensity and phase of an optical signal. In QKD, photons are generally used as a communication medium, and information is transmitted by superposing it on the quantum states of a single photon. An eavesdropper present on a transmission line can intercept the information by tapping photons being transmitted, or any other strategy. However, according to the Heisenberg's uncertainty principle, it is impossible to perfectly return a once-observed photon to the quantum state before observation. Therefore, a change occurs in the statistics of the reception data detected by an authorized receiver. By detecting this change, the receiver can detect the presence of an eavesdropper on the transmission line.
In the case of QKD utilizing the phase modulation of a photon, a sender and a receiver (hereinafter, referred to as Alice and Bob, respectively) constitute an optical interferometer, and Alice and Bob randomly perform phase modulation on each photon independently from each other. An output of “1” or “0” can be obtained depending on the difference between the depths of these phase modulations. Thereafter, Alice and Bob check against each other part of the conditions they used when measuring the output data, whereby Alice and Bob can ultimately share the same bit string.
However, in the case of implementing QKD in the real world, for lack of useful single-photon light sources, an alternative method is used in which the light intensity of an optical pulse generated by a laser diode (LD) for general communication is lowered to a single-photon level (weak coherent state) by using an optical attenuator. Therefore, the possibility remains that one pulse might include two photons or more, which works in favor of an eavesdropper. If an eavesdropping strategy called photon number splitting (PNS), described in Huttner, B., et al. “Quantum cryptography with coherent states,” Physical Review A, Vol. 51, No. 3, P. 1863, is used in particular, an eavesdropper can intercept the information on a bit with 100% certainty when one pulse corresponding to this bit includes two photons or more.
A measure for defending this PNS attack has been suggested. According to a QKD method employing a decoy state protocol disclosed in Hwang, W.-Y., “Quantum Key Distribution with High Loss: Toward Global Secure Communication,” Physical Review Letters, Vol. 91, No. 5, 057901 (2003), the intensity of each optical pulse is intentionally changed to any one of a signal-state intensity and a decoy-state intensity. Thereby, even if weak coherent light is used, since the mean number of photons per bit is intentionally changed beforehand, it is possible to monitor a change in the statistics of the number of received photons, which occurs when a PNS attack is present. Accordingly, it is possible to effectively prevent the PNS attack. A report on an experiment of QKD utilizing this technique is described in Zhao, Y., et al., “Experimental Decoy State Quantum Key Distribution Over 15 km,” quant-ph/0503192.
FIG. 2 is a block diagram showing a schematic configuration of a two-way QKD system described in Zhao et al. Here, Alice (sender/transmitter) 1510 and Bob (receiver) 1520 are connected through optical fiber 1530. An optical pulse in a multi-photon state is generated by a light source 1521 in Bob 1520 and sent to Alice 1510. The optical pulse arriving from Bob 1520 is detected by a photo detector (not shown) before the pulse enters a storage-use optical fiber 1513 in Alice 1510, and the result of this detection is notified to a variable optical attenuator 1514. The arriving optical pulse, after passing through the storage-use optical fiber 1513 and a phase modulator 1512, is reflected by a Faraday mirror 1511. The reflected optical pulse is phase-modulated (φA) by the phase modulator 1512 and then returns to the variable optical attenuator 1514 after passing through the storage-use optical fiber 1513.
Since the variable optical attenuator 1514 has been notified of the timing of the arrival of the optical pulse from the photo detector, the variable optical attenuator 1514 is driven at the timing when this optical pulse is returned by the Faraday mirror 1511, whereby the mean number of photons per optical pulse can be controlled. The mean number of photons per optical pulse is intentionally changed by using the variable optical attenuator 1514 in this manner, and then the optical pulse is sent to Bob 1520. Accordingly, by monitoring a change in the statistics of the number of received photons, Bob 1520 can detect the presence or absence of a PNS attack.
In such a decoy-state QKD system, however, a sender (Alice) in the conventional decoy-state QKD technology cannot perform both phase modulation and intensity modulation on each optical pulse at high speed with high reliability.
For example, in the decoy-state QKD system described in Zhao et al., the variable optical attenuator is disposed on the sender side (Alice) of the two-way QKD system, that is, at the returning end, whereby the mean number of photons per pulse is set. However, there are no variable optical attenuators that can operate at high speeds in the gigahertz range or so while ensuring the amount of attenuation for each of the polarization axes with accuracy. Therefore, the operable optical pulse repetition rate has a limit, resulting in a limit being imposed on the cryptographic key generation rate.
Additionally, in the two-way system described in Zhao et al., backscattered light of an optical pulse occurring on the outward journey (mainly due to Rayleigh scattering with non-changing wavelength) degrades the characteristics of the system as noise light. This noise can be ignored if the intensity of signal light is strong enough in comparison with the intensity of this backscattered noise light. However, since the signal light suffers optical loss inside the sender (Alice), the signal light might be lost in the backscattered noise light in an extreme case. To avoid the degradation of the characteristics of the system due to the backscattered noise light, it is necessary to make the optical loss inside the sender as small as possible. Therefore, it is important that as a small number of optical components as possible be used in the sender.
However, as shown in FIG. 2, the two-way system described in Zhao et al. has the configuration in which an optical pulse generated in the receiver (Bob) undergoes a round trip along the optical transmission line: going to the sender (Alice), returned there, and then coming back to the receiver (Bob). According to this configuration, inside the sender (Alice), which is the returning end, the optical pulse passes twice (on the outward and return journeys) through the optical components such as the variable optical attenuator, storage-use optical fiber, and phase modulator. Therefore, the optical loss inside the sender is large.
Moreover, in the case where the variable optical attenuator is provided in addition to the phase modulator, it is necessary to vary the timings when a train of optical pulses pass through these two devices in the outward and return journeys. Therefore, a timing design is difficult to make, and there arises a limit to the QKD system driving rate.